Internal oblique abdominis muscle (anatomy)

Internal oblique is one of the muscles of the abdominal wall. It originates from the:

• thoracolumbar fascia
• anterior two thirds of the iliac crest
• lateral two thirds of the inguinal ligament

Its fibres radiate widely from these origins around the abdominal wall. Its fibres insert into the:

• inferior borders of the most inferior three ribs
• aponeurosis of rectus sheath:
  • both sides join in the midline with the aponeuroses of the other abdominal wall muscles to form the linea alba
  • laterally, the superior fibres of internal oblique split at the border of the rectus abdominis muscle to contribute to both the anterior and posterior walls of the rectus sheath
  • inferior to the arcuate line, all fibres pass anterior to rectus abdominis
• pubic crest and pectineal line as the conjoint tendon:
  • inferior fibres of internal oblique pass over the spermatic cord within the inguinal canal
  • constitues part of the posterior wall of the canal at its medial end

Internal oblique acts to:

• support the abdominal wall
• raise intra-abdominal pressure eg for defaecation
• flex and rotate the trunk
• support the posterior wall of the inguinal canal

Internal oblique is innervated by:

• anterior primary rami of the most inferior five intercostal nerves (T7-T12)
• ilioinguinal nerve (L1) for the conjoint tendon